question_answer 51)

  Differentiate between (a) Open and closed circulatory system                  (b) Oviparity and viviparity (c) Direct and indirect development                        (d) Acoelomate and pseudocoelomate                  (e) Notochord and nerve cord                                   (f) Polyp and medusa

Answer:

  Ans.       For (a), (b) and (c) refer to 0. 1

(d) Acoelomate	Pseudocoelomate
The animals without coelom are called as acoelomates e.g., sponges, cnidarians, ctenophores and flatworms.	The animals in which body cavity is derived from the blastocoels of the embryo are called pseudocoelomates. e.g., roundworms.
(e) Notochord	Nerve Cord
Acelomate body plan (e) Notochord Notochord is the skeletal rod which lies lengthwise between the central nervous system and the alimentary canal or the gut. Vertebrates possess it at the young stage of development. It is a part of skeleton system In adults vertebrates, notocord is replaced by vertebral column.	Nerve cord is the solid strand of nervous tissue forming a part of central nervous system, especially in invertebrates.
(f) Polyp	Medusa
Polyp is sessile and cylindrical form in Aurelia. Polyp produces medusa asexually	Medusa is umbrella shaped and free swimming form in Aurelia. Medusa forms polyp sexually.